Hey instead of having to cut out the some of the beginning in order to make it loop you can just take a composition, double the time of what it would would originally be (e.g. 15s to 30s) and then do the work for the looping particles. Then do the same thing to loop the particles (toggle hold keyframe etc) but put the last keyframe for birth rate at 0 at 14:29 (e.g. on a 30 frame rate or the previous last frame depending on your frame rate) and leaving it at 14:29, pre compose, put a marker on the layer (At 14:29), duplicate the layer and move the layer that is on top over till the marker is at 0:00 so now you have a looping particle system and then you can just change the comp duration back to 15s as it would originally be. I find this convenient but either way it all works lol. =D